Theoretical and experimental investigation of Kramers-Henneberger states in alkaline and noble gas atoms

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The main objective of this project is to provide direct spectroscopic evidence for the bound states of a free electron an unusual quantum state created by the concerted actionof the attractive core potential and a strong laser field. Electrons in such states respond to the laser field almost like free electrons, yet on average they remain bound to the ionic core. The states are often referred to as the Kramers-Henneberger (KH) states, after first theoretical predictions made about 50 years ago. For many decades, the KH states looked like a purely theoretical concept of academic interest. However, today there is mounting indirect experimental evidence suggesting that these states are ubiquitous and can emerge almost any time an atom or a molecule is exposed to sufficiently intense infrared laser fields. The combined action of the attractive core potential and the laser electric field creates a potential barrier through which a bound electron can escape. As the laser intensity increases, the top of the potential barrier can descend below the energy of a bound electronic state. Yet, the state does not necessarily become free. While the electron oscillation amplitude in such a state reaches some ten angstroms, it remains stable against ionization. Crucially, this situation is typical for all excited atomic states exposed to infrared laser fields with intensities in mid-1013 W/cm2 and higher. Such restructuring of the atomic spectrum has important implications for nonlinear light-matter interaction, including such processes as laser filamentation. The emergence of Kramers-Henneberger states inside a laser filament is extremely likely, and their role in the filamentation process can be substantial. One of the key goals of our project is to investigate the emergence of these states, their role in the filamentation process, and provide conclusive, direct spectroscopic evidence of their existence. Our goal is to provide two complementary spectroscopic observations, based on angle-resolved photo-electron spectroscopy of isolated atoms and transient absorption spectroscopy in laser filaments, to resolve this intriguing situation.